Generally speaking, neither clothing nor temperature affects the amount of the metabolism during exercise. They influence only the quantity of water eliminated in the perspiration, in the effort of the body to 'maintain its normal temperature through physical regulation. It is evident from Rubner's details of the water excretion that at a low temperature the extra heat production during mechanical exercise is lost by radiation and conduction. Rubner explains that the slight increase in the excretion of water above that lost while at rest is due to its increased evaporation through increased respiratory activity. At a higher temperature conduction and radiation become insufficient to cool the body, and a large proportion of the loss of heat takes place at the expense of the evaporation of sweat.

In hot, moist climates, however, the cooling of the body through the evaporation of moisture becomes difficult, and this is especially pronounced in the case of fat people (p. 147), who with difficulty discharge the heat produced within them. Broden and Wolpert2 show the effect of the action of temperature and humidity on the metabolism of a fat man, weighing 101 kilograms, who executed the same amount of mechanical work under various conditions of experimentation. The work was light, being 5375 kilogrammeters per hour. The results were as follows:

1 Voit: "Zeitschrift fur Biologie," 1878, xiv, 152.

2 Broden and Wolpert: "Archiv fur Hygiene," 1901, xxxix, 298.

Effect Of Work, Temperature, And Humidity On The Metabolism Of A Fat Individual

This individual was the same already mentioned (p. 147), and the explanation given there is equally applicable here. In a dry climate the same amount of mechanical work may be accomplished by a fat person at both 200 and 300 without changing the metabolism. At a temperature of 370 the metabolism rises, for the cooling power of the evaporating sweat does not seem sufficient to act through the dense covering of fat. This action is intensified in moist air, where the evaporation of water is hindered. Under these latter conditions the small amount of work was accomplished only at the expense of great discomfort and profuse perspiration.

The obese, therefore, work under great disadvantage in a hot, and especially in a hot and moist, climate. The profuse perspiration explains their desire for water to drink.

In the early experiments of Pettenkofer and Voit, already cited, it was shown that work did not raise the protein metabolism even in starvation, and that the source of the power appeared to be the increased combustion of the non-nitrogenous fat.

In other experiments a slight rise in the nitrogen metabolism, continuing into the day following work, has been noted. The protein metabolism, however, is not sufficient to yield the energy necessary for a hard day's work. In the well-known experiments of Fick and Wislicenus1 the authors climbed the Faulhorn, in Switzerland, a mountain 1956 meters high. The product of their weight into the height to which they raised themselves gave them a close approximation to the amount of the work done. The experimenters took their last nitrogenous food seventeen hours before starting on their walk. They climbed for six hours and collected the urine of this period and that of seven hours thereafter. Their results were as follows:

The work accomplished represents three times the energy liberated from the protein metabolism of the time. The output of energy as measured above was not all the increase in the amount of mechanical energy during the period, for the heart and respiratory muscles acted with greater force, and energy was expended by swinging the arms and by friction on the road.

The fact observed by Pettenkofer and Voit that protein metabolism may not be appreciably affected during mechanical work has been abundantly confirmed by Krummacher.2 A porter, weighing 79 kilograms, was given a diet containing 3700 calories, 14.28 grams of protein nitrogen, and a large amount of carbohydrate. The man turned a dynamometer and produced 402,000 kilogrammeters of work. The slight increase in protein metabolism could have yielded but 3 per cent, of the energy required for the work. Krummacher states that protein metabolism may increase during work only when the non-nitrogenous fat and carbohydrates become less available in metabolism. We have already seen that protein metabolism rises in the absence of carbohydrates. It may be that with the exhaustion of carbohydrates during exercise a period ensues when the loss of their influence leads to an increased protein destruction. The larger the quantity of carbohydrates given, the less marked would be this influence. It is interesting in this connection that soldiers when starting on a march may have a high respiratory quotient (indicating the combustion of carbohydrates), which falls at the end of the march (fat combustion) and which may remain lower than at first, even on a day following the march.1 The fact that mechanical work may be accomplished at the expense of an increased combustion of fat and carbohydrates should not cause one to forget that protein may become the sole source of energy in the body. It has already been shown that a fasting animal, after burning all his fat, may maintain his life on protein alone (see p. 101), and that Pfluger kept a dog in active condition on meat alone. As protein may yield 58 per cent, of sugar this substance may still be the principal source of energy.

1 Fick and Wislicenus: "Myothermische Untersuchungen," 1889. 2 Krummacher: "Zeitschrift fur Biologie," 1896, xxxiii, 108.